1. Field of the Invention
The invention relates to a method and a circuit arrangement for adjusting the level and dynamic range of an audio signal in a transmission system and particularly in a mobile station.
2. Description of the Prior Art
There are several factors related to mobile telephone systems that deteriorate the quality of a speech signal transmitted via a transmission path. A received audio signal consists of a "payload" signal, such as speech, and noise. The latter may be electrical noise from a transmitter or transmission channel, interfering signals connected to the transmission channel from external sources or it may be caused by acoustic noise in the information source environment. The received noise signal and ambient noise in the environment of the receiving equipment reduce the intelligibility of the transmitted speech reproduced by the mobile phone. Furthermore, the levels of aforementioned noise signals and the level of the speech signal may vary, which makes it even more difficult to follow the reproduced speech.
A signal received by a mobile phone typically has a large dynamic range, ie. the difference between the highest and lowest signal level during a certain period of time is great. When the dynamic range of the received signal is large, it is hard to discern speech reproduced at the lowest signal levels from the noise signal and background noise, and at the highest signal levels the speech becomes disturbingly loud. In addition, at high signal levels the speech signal may be distorted in the receiver circuits, especially in the output amplifier and loudspeaker.
It is known several methods to attenuate interference and noise. U.S. Pat. No. 5,241,689 discloses a mobile telephone system in which audio signals from various sources arriving in a base station are converted to a predetermined amplitude level before the signal is sent to the radio channel. The intention of this arrangement is to make maximal use of the radio channel dynamic range and it does not essentially solve the other problems mentioned above.
It is known from patent documents EP 113615 and EP 514813 to compress the signal to be transmitted, or to narrow down the dynamic range of the signal to be transmitted before transmission and to expand it at reception with a corresponding reverse function. The compression algorithm parameter information is sent by means of a separate signal. The intention of this solution is to reduce the effect of noise accumulated in the signal transmitted on the transmission path especially in the uplink direction, ie. when transmitting a signal from a mobile phone to a fixed communication network. However, the method does not solve the other problems mentioned above.
From patent document EP 0 600 164 it is known a method wherein the compression parameters of a compression performed to a signal are adjusted in response to the noise level and the level of the signal from the microphone. The intention of the arrangement is to attenuate exceptionally strong microphone signals before transmission. This method improves the quality of the signal transmitted by a mobile phone but it cannot improve the reproduction quality of the signal received by a mobile phone.
U.S. Pat. No. 5,070,527 describes a method in which the level of a speech signal received from a transmission channel is compressed between two predetermined extremes. The system is intended to be used in conjunction with a headset in telephone exchanges to limit the reproduced speech signal below a maximum level determined by health authorities. The system uses for compression a feedback in which a peak value is defined for the speech signal, the defined value is compared to a predetermined limit value and the input signal amplification is controlled such that the resulting signal stays between the set limit values. In addition, the system includes a time constant according to which a signal at its upper limit value is reduced to the lower limit value. When the system uses peak value measurement instead of power measurement, the output signal level is limited below a predetermined value in all situations. This arrangement makes it easier to follow a received signal in a noisy environment and prevents the reproduction of signal levels too high. However, the arrangement does not solve problems caused by noise included in a received or transmitted signal.
U.S. Pat. No. 5,208,866 discloses an arrangement relating to a car stereo equipment, wherein the sound volume of the receiver is automatically adjusted in response to the ambient noise level. If this solution were applied in a mobile communication system, it would have the disadvantage that if the level of noise included in the received signal were high, the speech reproduced by a mobile station would in any case be masked by noise included in the received signal and by ambient noise.
Thus, none of the prior art methods mentioned above can solve all aforementioned problems related to the reproduction of an audio signal. Furthermore, combining said prior art solutions would result in a complex implementation which would result in a significant increase in the manufacturing costs when applied to mobile phones, for example.